1. Field of Application
The present invention relates to a dual-transformer type of DC-to-DC converter, and in particular to an improved DC-to-DC converter providing high efficiency of conversion.
2. Description of Prior Art
Types of DC-to-DC converter which utilize two transformers have been proposed in the prior art, for example in Japanese patent application first publication No. 2003-102175 (referred to in the following as reference document 1), U.S. Pat. No. 5,291,382 (referred to in the following as reference document 2), Japanese patent application first publication No. 2005-51994 (referred to in the following as reference document 3), and Japanese patent application first publication No. 2005-51995 (referred to in the following as reference document 4). Reference documents 3 and 4 are each by the assignees of the present invention. In the DC-to-DC converters disclosed in each of reference documents 2, 3 and 4, during each interval in which electrical power is being outputted by the secondary side of a first one of the two transformers due to the transformer action of that transformer, the second transformer is storing magnetizing energy to enable a succeeding transformer action by that second transformer. During each interval in which power is being outputted from the secondary side of the second transformer, i.e., by the aforementioned succeeding transformer action, the first transformer is releasing magnetizing energy to enable a succeeding transformer action.
Due to the fact that primary windings of the two transformer are connected in series, the released manetizing energy of a first one of the transformers (i.e., manetizing energy stored in a primary windings of that transformer) is transferred to a primary winding of the second transformer, when electrical power is transferred to the secondary winding of the second transformer by transformer action from that primary winding of the second transformer. Thus due to the release of the stored manetizing energy, energy losses can be reduced. Due to the fact that the transformer actions of the two transformers (i.e., outputting of current to the secondary windings) and releasing of the magnetizing energy occur in successive alternation, the output currents from the respective secondary windings can be combined to obtain a substantially DC output current from the DC-to-DC converter.
Thus with each of the DC-to-DC converters of reference documents 2 to 4, the two transformer operate in alternation to produce an output voltage that is substantially a DC voltage, with only a small amount of ripple. In the case of the DC-to-DC converters of reference documents 2 and 3, a capacitor is used to transfer current between the primary windings and the primary-side power source, with charge storage by the capacitor serving to achieve a “soft switching” effect. The switching losses which occur in the primary-side switching elements (due to successive interruptions of the primary-side current) are thereby reduced so that a high efficiency of power transfer can be achieved.
Reduction of the losses that occur in a high-power type of electronic apparatus enables the cooling mechanism of that apparatus to be made smaller and more light in weight, and also enables the energy lost by performing cooling of the apparatus to be reduced. This is especially valuable in the case of an apparatus that is to be installed in a vehicle, since the available space for installing equipment is limited.
However with the DC-to-DC converters of reference documents 3 and 4, since high voltages are applied across the terminals of the switching elements in the high-voltage side of the apparatus, there is the disadvantage that it is necessary to use a plurality of expensive types of switching elements having a high breakdown voltage level. Furthermore, a switching element (more specifically, semiconductor device that is operated as a switching element) having a high breakdown voltage utilizes a relatively thick layer of low impurity-concentration material as a layer which withstands high voltages. As a result, the ON-state resistance of such a switching element is substantially greater than that of a switching element that is designed to only withstand low voltages. Hence, a switching element having a high breakdown voltage level has a high level of switching loss.
In general, there is a fixed limit on the maximum permissible level of loss for each semiconductor switching module (formed of one switching element or a plurality of switching elements connected in parallel). If switching elements having a high breakdown voltage level are utilized, then it becomes necessary to reduce the current density in each switching element, in view of the increased ON-state resistance of such a switching element. Thus in order to achieve a required level of current, it becomes necessary to connect a plurality of expensive switching elements (having a high breakdown voltage level) in parallel to constitute a switching module, instead of utilizing only a single switching element. Hence the manufacturing cost of the DC-to-DC converter becomes increased.